System for copy protection of an information carrier

ABSTRACT

The present invention relates to a system for copy protection of an information carrier, said system comprising a diffractive layer for delivering a speckle pattern when illuminated by a light source, a spatial filter, which is aligned with respect to the diffractive layer, for delivering a filtered optical signal from the speckle pattern and a detector array for delivering, when illuminated by said filtered optical signal, an electrical signal. Said system further comprises means for computing a cryptographic key from the electrical signal, and means for decrypting encrypted data contained in the information carrier from the cryptographic key. It finds its application in copy protection of content carriers such as optical discs or in smart cards.

FIELD OF THE INVENTION

The present invention relates to a system for copy protection of aninformation carrier.

The present invention also relates to an information carrier for use insuch a system.

It relates to a method of and a device for reading such an informationcarrier.

It finally relates to a method of manufacturing a cryptographic key inhardware to prevent copying the information carrier in accordance withthe invention.

It finds its application in copy protection of content carriers such asoptical discs or in smart cards.

BACKGROUND OF THE INVENTION

Physical one-way functions have already been proposed as a basis forcryptography, copy protection or identification cards. According to theknown prior art, a diffractive structure is illuminated with anarbitrary wave front, and the resulting speckle pattern is detected by adetector, for instance a CCD camera. From the speckle pattern acryptographic key is created, which uniquely identifies the diffractivestructure. The robustness of such a system depends strongly on thealignment between the diffractive structure and the detector.

The paper entitled “Physical one-way functions”, Science, 20 Sep. 2002,p. 297 shows the use of diffractive structures for realizing one-wayfunctions for cryptography purpose. Said paper is based on the detectionof the speckle pattern by a high resolution CCD detector, which israther expensive. And some problems can be expected from alignmentbetween the light source, the diffractive structure and the detector.

SUMMARY OF THE INVENTION

It is an object of the invention to propose a system for copy protectionof an information carrier that is less complex than the solution of theprior art while keeping an equivalent efficiency.

To this end, the system in accordance with the invention comprises:

a diffractive layer for delivering a speckle pattern when illuminated bya light source,

a spatial filter, which is aligned with respect to the diffractivelayer, for delivering a filtered optical signal from the specklepattern,

a detector array for delivering, when illuminated by said filteredoptical signal, an electrical signal,

means for computing a cryptographic key from the electrical signal, and

means for decrypting encrypted data contained in the information carrierfrom the cryptographic key.

As a consequence, the whole cryptographic structure, i.e. thediffractive layer, the spatial filter and the detector array, can beseen as one “chip”, which gives a unique response to each incoming wavefront. By rapidly varying the incoming wave front the uniqueness of agiven diffractive structure can safely be identified.

According to a first embodiment of the invention, the diffractivestructure, the spatial filter and the detector array are combined in onepiece of hardware.

According to another embodiment of the invention, the detector is partof a read-out unit, the other parts of the cryptographic structure beingcombined in one piece of hardware.

According to another embodiment of the invention, the detector and thespatial filter are part of the read-out unit, while the diffractivestructure is part of the content carrier. The spatial filer is then madeof a reversible photosensitive material and is created every time acarrier is inserted into the read-out unit.

The present invention also relates to a method of manufacturing acryptographic key, comprising the steps of:

holographic exposing a layer of photopolymer for creating a diffractivestructure,

flood exposing said photopolymer layer to polymerize said diffractivestructure,

illuminating a first photosensitive material with a light source throughthe diffractive structure for forming a spatial filter having apredetermined pattern, an activation of said photosensitive materialbeing performed when an intensity of a speckle pattern delivered by thediffractive structure for a given wave front of the light source ishigher than a predetermined threshold.

Said method is adapted to make the diffractive structure, the spatialfilter and possibly the detector array in a single step by opticalillumination of photosensitive materials. This guarantees properalignment of the detector, the spatial filter and the diffractivestructure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic view of the cryptographic structure in accordancewith the invention,

FIG. 2 illustrates the operating principle of the cryptographicstructure in accordance with the invention, and

FIG. 3 illustrates the method of manufacturing a cryptographic key.

DETAILED DESCRIPTION OF THE INVENTION

The present invention aims at proposing a cheap and efficient system forcopy protection of an information carrier based on the diffractionprinciple. The information carrier is, for example, an optical disc,such as a Small Factor Format Optical SFFO disc or a Blu-ray disc, butit will be apparent to a person skilled in the art that the presentinvention is also applicable to other type of information carrier, suchas, for example, magnetic disks or smart cards.

FIG. 1 is a layout of an information carrier in accordance with theinvention and FIG. 2 illustrates the operating principle of saidinformation carrier. Such an information carrier comprises a diffractivelayer 1 for delivering a speckle pattern 11 when illuminated by a lightsource 10, a spatial filter 3 for delivering a filtered optical signal31 from the speckle pattern and a detector array 4 for delivering, whenilluminated by said filtered optical signal, an electrical signal, fromwhich a cryptographic key can be generated. It also comprises a spacer 2for separating the diffractive layer from the spatial filter. Thisspacer plays a role of propagation medium. Its width is larger than thewavelength of the light source and smaller the width of the diffractivestructure. These different structures are here combined in one piece ofhardware.

The present invention proposes to make the diffractive layer and thespatial filter in a single step by optical illumination ofphotosensitive materials in order to guarantee a proper alignment of thespatial filter and the diffractive structure and an efficient operationof the cryptographic structure.

According to a first embodiment of the invention, the system inaccordance with the invention comprises a simple detector array withlarge pixels. As a very sensitive system is required in order to detectif a hacker tries to clone the cryptographic key, it is necessary tocompensate the poor performance of the detector array. This is donethanks to a very accurate spatial filter based on the use of one-wayphysical Gabor hash function for example.

The information layer in accordance with the invention gives a uniqueresponse to each incoming wave front emitted by a light source. Notethat even when the responses from a large set of incoming wave frontsare known, this is not enough to predict the response to a wave frontoutside this set. As a consequence, a hacker cannot create a diffractivestructure, which has, in combination with the spatial filter, the samesignal on the detector.

As shown in FIG. 1, the cryptographic key is read out directly from thedetector array. To this end, the system for copy protection furthercomprises a device for reading such an information carrier. Said devicecomprises means for computing a cryptographic key from the electricalsignal delivered by the information layer, and means for decryptingencrypted data contained in the information carrier from thecryptographic key.

Such a device is consequently adapted to identify from the associatedcryptographic structure the characteristic information present on theinformation carrier, to derive from it the cryptographic key, and todecrypt the user-information recorded on the information carrier. Arecorder attempting to clone an original data carrier, i.e. to duplicatethe user-information contained in an original data carrier, on a seconddata carrier will not be able to record onto the second data carrieralso the characteristic information, like the one present on theoriginal data carrier, necessary to decrypt the user-information. As aconsequence the user-information contained in the second data carriercannot be decrypted and played.

Security comes from the fact that there is a “one-way” relation betweenthe diffractive layer and the cryptographic key. A security module isthen able to verify the relation between the cryptographic key and theincoming wave front, thereby determining uniquely the identity of thestructure.

The diffractive structure is made of photopolymers. They consistbasically of one or more monomers or a combination of polymer andmonomer, a binder functioning as an inactive component, and aninitiation system. The medium is illuminated by a three-dimensionalspatial pattern of high and low intensity. In the areas of highintensity, polymerization of the monomer is photo-initiated. Severalmechanisms subsequently occur that cause a refractive index modulationconsisting in changes that occur in the molecular electronic structure,density changes that occur upon polymerization, spatial segregation ofthe components within the system.

During a holographic exposure, spatial variations in polymerizationrates induce spatial variations in monomer consumption. Due to thislocal depletion of the monomer and the additional shrinkage resultingfrom the polymerization, diffusion of monomer from the dark regions tothe bright regions occurs. This process continues until the monomer isdepleted completely in all regions or until the mobility is decreased,due to the increasing molecular weight of the system, to such an extentthat monomer diffusion is halted. The spatial segregation of the variouscomponents results in refractive index differences when the refractiveindices of both components vary significantly, and contributes to theoverall refractive index modulation.

After the holographic exposure, the diffractive structure is illuminatedwith a flood exposure to fixate the structure by complete polymerizationof any residual monomer. Hence, without component segregation,post-exposure illumination would result in a featureless, uniformlypolymerized sample with no holographic activity.

The prediction of the result of the polymerization process is extremelydifficult to determine. The diffusion constants of the individualcomponents not only vary with polymerization time as a result of thechanging viscosity of the environment, but also vary spatially, as aresult of the spatial distribution of the light.

The non-linearity and unpredictability of the diffractive layerresulting from the polymerization process make it interesting forcryptography application, as it can not be predicted which diffractivestructure is formed by a given illuminating wave front.

As the detector array has to be cheap and easy to manufacture, itslateral resolution is relatively poor. In order to compensate for that aspatial filter comprising a binary mask with very tiny holes is neededto enhance the lateral resolution significantly. The spatial filter hasto be very well aligned laterally but also in the depth direction withrespect to the diffractive structure. That is why the spatial filter ismade or activated at the same step as the diffractive structure is fixedby flood-exposure.

This functionality can be provided by various materials, changing theiroptical properties from reflecting or absorbing to transparent uponstrong irradiation. These materials are, for example, standard materialsin optical storage, such as organic dyes, metal layer on organics, phasechange materials, or suicides.

Alternatively, inorganic bi-layer materials proposed as resist can beused. These materials are selected from the group consisting of pairs:As—Pb, Bi—Cd, Bi—Co, Bi—In, Bi—Pb, Bi—Sn, Bi—Zn, Cd—In, Cd—Pb, Cd—Sb,Cd—Sn, Cd—Ti, Cd—Zn, Ga—In, Ga—Mg, Ga—Sn, Ga—Zn, In—Sn, In—Zn, Mg—Pb,Mg—Sn, Mg—Ti, Pb—Pd, Pb—Pt, Pb—Sb, Sb—Sn, Sb—Ti, Se—Ti, Sn—Ti, andSn—Zn. For example, the bi-layers Bi—In or Bi—Sn can be used for theiroptical properties. Such a deposited bi-layer system is defined by highreflection and more or less “zero” transmission. After alloyinginitiated thermally or by intense light irradiation the bi-layer systemappears to be transparent.

FIG. 3 illustrates the principle of structuring or “activating” thespatial filter properly. Under illumination by a light source, thediffractive structure provides a speckle pattern at a certain distancedepending on the diffractive structure but also on the wave front of thelight (wavelength, angle, divergence, phase, polarization). At theselocations where the intensity 110 of the speckle pattern delivered bythe diffractive structure for a given wave front of the light sourcereaches a certain threshold value materialized with the line T, thematerial is transformed becoming transparent. Below this threshold, thephotosensitive material stays non-transparent. By this way aone-dimensional or two-dimensional binary mask comprising transparentparts 3 a and non-transparent parts 3 b is generated. Furthermore, thispattern is perfectly aligned with respect to the diffractive structure.

The choice of the proper detector is arbitrary for the working principleof the invention. However, it is preferred to use a relatively cheapapproach, because the detector can be a part of the information carrier,such as for example copy protected optical disc produced in highvolumes, and in large volume production low prices are an essentialrequirement. Said detector has large pixels 41 to 44 and consequently aresolution lower than the spatial filter.

It is therefore proposed to make use of a patterned photoelectric layerproviding an electrical signal, which needs to be read out to get theoptical cryptographic key. A combination, for example a correlation, ofthe electrical signal delivered by the photoelectric layer and of theoptical signal delivered by the diffractive structure makes a uniqueoptical cryptographic key, which gives access to the content of theinformation carrier, said content being encrypted with said key.

According to an embodiment of the invention, the photoelectric layer ispatterned using a segmental activating or deactivating of said layer.Thanks to this segmentation the electrical signal corresponding to thepredetermined pattern is generated when illuminated with an opticalsignal. The predetermined pattern can be written in the photoelectriclayer, for example during manufacturing of the disc by patterned UVirradiation in an oxygen environment. This results in a locallyhigh-ohmic behavior, which does not give rise to any current underillumination.

Alternatively, the electrodes could also be patterned. For instance,thin metal films on silicon could be used, acting as metallicelectrodes. After a local heating or after an irradiation, silicides areformed. Said silicides are typically defined by high-ohmic orsemi-conducting behavior.

According to another embodiment, the photoelectric internal layer issegmented in activated and deactivated areas and one electrode issegmented in segments.

The photoelectric layer is made of existing materials and stackcombinations used in solar cells, for instance amorphous silicon asdescribed by I. Garner in “Communications-International”, vol. 16, no. 3(1989) 73.

It can also be made of photoelectric tungsten disulfide WS₂ as describedin “Solar Energy Materials & Solar Cells”, by C. Balif, M. Regula, F.Levy, 57 (1999) 189, of copper, indium or gallium selenide, cadmiumdisulfide, cadmium diselenide, gallium arsenide, aluminum galliumarsenide.

The photoelectric layer can be made of organic solar cell materialsbased on conjugated polymer. It can be, for example, conjugatedpolymer/methanofullerene as described in “2.5% Efficient Organic PlasticSolar Cells”, by S. E. Shaheen C. J. Brabec, N. S. Sariciftci, F.Padinger, T. Fromherz, J. C. Hummelen, Applied Physics Letters, vol. 78,no. 6 (2001) 841, or conjugated polymer/conjugated polymer, conjugatedpolymer/organic molecule, organic molecule/organic molecule, conjugatedpolymer/inorganic oxides, selenides and sulfides, organicmolecule/inorganic oxides, selenides and sulfides.

The photoelectric layer can be based on carbon nanotubes as described byE. Kymanskis et al. Applied Physics Letters, vol. 80, no. 1 (2002) 112,or nanowires, preferably carbon nanotubes or metal oxide nanotubes.

The efficiency of these materials used as solar cells is relatively lowdue to the broad solar spectrum. However the invention is related tofuture generation of optical storage, which will make use of relativeshort wavelength, such as 405 nm for blu-ray disc, where quantumefficiency of up to 60% can be reached using proper dye materials.

It is to be noted that the diffractive layer, the spatial filter and thephotodetector array can all be produced by photosensitive processes.

Therefore the whole structure can be developed in a single device,eliminating all alignment errors.

Alternatively, the detector is part of the read out unit, i.e. thedetector is not directly part of the optical cryptographic structure. Inthat case, the essential parts of the cryptographic structure being thediffractive structure, the spacer and the spatial filter are combined inone piece of hardware and aligned precisely with respect to each other,whereas the detector, which is part of the reader, is a CCD or a CMOSdetector for example.

An advantage of such a solution is a further cost reduction of theinformation carrier.

In a further embodiment, the detector and the spatial filter are part ofthe read out unit, while the diffractive structure is part of theinformation carrier. The spatial filter is thus created every time aninformation carrier is inserted into the read out unit. To this end, thespatial filter is made of a photosensitive reversible layer, such as forinstance an AgCl/CuCl layer, which blackens under strong illuminationand gets back to a transmission state in a dark environment. Thediffractive structure on the information carrier is stronglyilluminated, creating a gray-scale absorption pattern on the spatialfilter. Subsequent illumination of the diffractive structure with a weaklight source results in a diffraction pattern that is partly transmittedthrough the spatial filter onto the detector array. By changingparameters such as angle, wavelength, or position of the weak lightsource, a series of checks can be performed. It is to be noted that theproposed spatial filter returns to its initial transmission state afterrelatively short time.

The main advantage of this embodiment is the fact that the diffractivestructure is the only part of the system present on the informationcarrier, the other parts being in the read out unit. This makes thisembodiment very cheap. Still, the read out unit part and the informationcarrier part of the optical detection system are perfectly aligned justas they are in the embodiments described above. Furthermore, byproducing the read out unit part of the optical detection system on onechip, no signal can be monitored.

In addition, the patterning of the spatial filter and the cryptographicoptical key read out procedure can either be performed on a moving, forexample rotating, or a stationary information carrier.

Any reference sign in the following claims should not be construed aslimiting the claim. It will be obvious that the use of the verb “tocomprise” and its conjugations do not exclude the presence of any othersteps or elements besides those defined in any claim. The word “a” or“an” preceding an element or step does not exclude the presence of aplurality of such elements or steps.

1. An information carrier comprising: a diffractive layer made ofphotopolymers, for delivering a speckle pattern when illuminated by alight source, a spatial filtering layer including a binary mask made ofa photosensitive material, for delivering a filtered optical signal fromthe speckle pattern, said spatial filtering layer being aligned withrespect to the diffractive layer, and a detection layer for transformingsaid filtered optical signal into an electrical signal, from which acryptographic key is generated.
 2. An information carrier as claimed inclaim 1, wherein the detection layer is made of a patternedphotoelectric material.
 3. An information carrier as claimed in claim 1,further comprising a spacer for separating the diffractive layer fromthe spatial filtering layer, said spacer having a width which is largerthan the wavelength of the light source and smaller than the width ofthe diffractive layer.
 4. A device for reading an information carrier asclaimed in claim 1, said device comprising: means for computing acryptographic key from the electrical signal delivered by the detectionlayer, and means for decrypting encrypted data contained in theinformation carrier based on the cryptographic key.
 5. An informationcarrier comprising: a diffractive layer made of photopolymers, fordelivering a speckle pattern when illuminated by a light source, and aspatial filtering layer including a binary mask made of a photosensitivematerial, for delivering a filtered optical signal from the specklepattern, said spatial filtering layer being aligned with respect to thediffractive layer.
 6. An information carrier as claimed in claim 5,further comprising a spacer for separating the diffractive layer fromthe spatial filtering layer, said spacer having a width which is largerthan the wavelength of the light source and smaller than the width ofthe diffractive layer.
 7. A device for reading an information carrier asclaimed in claim 5, said device comprising: a detector array fortransforming the filtered optical signal into an electrical signal,means for computing a cryptographic key from said electrical signal, andmeans for decrypting encrypted data contained in the information carrierfrom the cryptographic key.
 8. A device as claimed in claim 1, whereinthe detector array is made of a patterned photoelectric material.
 9. Adevice for reading an information carrier comprising a diffractive layerfor delivering a speckle pattern when illuminated by a light source,said device comprising: a spatial filter for delivering a filteredoptical signal from the speckle pattern, said spatial filter including abinary mask made of a reversible photosensitive material such that saidbinary mask is created every time an information carrier is insertedinto said device, a detector array for transforming the filtered opticalsignal into an electrical signal, means for computing a cryptographickey from said electrical signal, and means for decrypting encrypted datacontained in the information carrier from the cryptographic key.
 10. Amethod of manufacturing an information carrier as claimed in claim 1,said method comprising the steps of: holographic exposing a layer ofphotopolymer so as to create a diffractive structure, illuminating atthe same time said photopolymer layer so as to polymerize saiddiffractive structure, and a layer made of photosensitive materialthrough the diffractive structure so as to form a spatial filter havinga binary mask including activated and non-activated areas, an activationof said photosensitive material being performed when an intensity of aspeckle pattern delivered by the diffractive structure for a given wavefront of the light source is higher than a predetermined threshold.